Helicopters generally have utilized mechanical linkages leading from a pilot control stick to actuator inputs for controlling the location and attitude of a swashplate. The swashplate is then coupled to pitch horns on rotor blades to control cyclic pitch and collective pitch. Variations in cyclic pitch are produced by tilting the swashplate. Variations in collective pitch are produced by raising and lowering the swashplate. The position and attitude of a swashplate may be controlled by three actuators connected to three points on the swashplate. Electrical sensors have been utilized to sense pilot inputs for producing control signals in a servo system which in turn drives swashplate actuators.
Military aircraft are vulnerable to damage by enemy fire. Because of such exposure, military specifications for a control system require continued operability, even though a given control channel fails, is damaged or otherwise is placed out of action.
Any three actuators may determine the plane and position of the swashplate. More than three actuators may provide for operating redundancy. In such case, the failure of mechanical hardware as upon ballistic impact can be tolerated to a degree depending upon the extent of redundancy.